1. Field of the Invention
The present invention relates to a method for manufacturing an electrode foil which is used as an anode foil of an electrolytic capacitor.
2. Description of Related Art
A metal foil for aluminum electrolytic capacitor is manufactured using a foil, as a starting material, which is formed by rolling aluminum having a high purity of 98% or more.
FIG. 24 is a schematic view illustrating a cross-section of an anode foil (aluminum foil 2) of an aluminum electrolytic capacitor.
The aluminum foil 2 formed by a rolling treatment is immersed in an acidic solution or is electrolytically etched in an acidic or an alkaline solution to form numerous number of etching pits 4 on the surface, so that an effective surface area of the aluminum foil 2 is increased. This etching treatment is also called a surface roughing treatment.
By the etching method described above, the magnification (ratio of an effective foil area obtained by surface roughing to a smooth foil area) per unit area is several hundreds times, that is, approximately 300 to 400 times.
In addition, by a chemical conversion treatment which anodizes an aluminum foil having an increased surface area as described above to form an extremely thin oxide film, a dielectric layer is formed which separates an anode composed of an aluminum foil and a practical cathode composed of an electrolytic solution (cf. “Electrolytic Solution Cathode Aluminum Electrolytic Capacitor” by Isaya NAGATA of Japan Capacitance Industrial Co., Ltd., published in 1997).
In an aluminum electrolytic capacitor as described above using a metal foil as an anode foil, the surface area of an electrode is extremely large, and a dielectric layer is extremely thin. Accordingly, an electrostatic capacitance per unit area (unit area obtained when the electrode foil is assumed to be smooth) of the electrode is large, and hence an aluminum electrolytic capacitor having a small size and a large capacitance can be advantageously obtained.
However, the reduction in size of electronic devices demands further reduction in size and increase in capacitance of electrolytic capacitors in the market.
In order to increase the capacitance of an electrolytic capacitor, it is effective to increase an effective surface area of an electrode by performing an intensive etching treatment performed on an aluminum foil.
FIG. 25 illustrates a schematic view showing a cross-section of the aluminum foil 2 processed by an intensive etching treatment. When the intensity of an etching treatment is excessively increased, the etching pits 4 will be formed to reach a deep area of the aluminum foil 2, as a result, the metal foil becomes brittle, and the strength thereof is decreased.
Accordingly, heretofore, when it is intended to increase the capacitance of an electrolytic capacitor, the thickness of an aluminum foil used as a starting material is increased, and an etching treatment is then performed, so that the surface area of the metal foil is increased while the foil strength is maintained.
By the method described above, the capacitance per unit area is increased; however, since a thick aluminum foil is used, the volume obtained after winding is unfavorably increased in the case of a winding type electrolytic capacitor. This increase in volume is against the requirement for reduction in size.
As with the case described above, when a laminated type solid electrolytic capacitor is formed, the volume is also increased when aluminum foils are laminated to each other, and this increase in volume is against the requirement for reduction in size.
That is, it is difficult to meet further requirements from the market for increase in capacitance and reduction in size by a conventional etching method as a surface roughing method.